Known probe stations, also known as wafer probers, for testing semiconductor substrates on wafers under defined conditions consist of a wafer chuck 1, which is connected in some cases with an add-on plate 2 mounted on its top surface and a 4axis manipulator stage 3, connected with the wafer chuck 1. The 4axis manipulator stage 3 can move the wafer chuck 1 relative to a probe needle arrangement 4 assembled above the wafer chuck 1 and the machine frame 5 (FIG. 1). The 4axis manipulator stage can move the wafer chuck 1 and the add-on plate 2 along x, y, z and Theta directions as shown schematically in FIG. 1.
At least the wafer chuck 1 and the add-on plate 2 are often arranged within a chamber 6, providing the required environmental conditions for testing. The accessibility of the wafer chuck surface within the chamber 6 is limited for the operator. This leads to difficulties during loading and unloading of a wafer 12 on add-on plate 2 and occasionally required calibration substrates 7, which are located beside the wafer on the add-on plate (FIG. 1) or the wafer chuck 1 (FIG. 1a).
The wafer 12 is fastened on the wafer chuck 1 or alternatively on the add-on plate 2 (FIG. 1) as well as the calibration substrate 7 is fastened on the wafer chuck 1 (FIG. 1) or the add-on plate 2 by vacuum suction. Therefore the wafer chuck 1 and the add-on plate 2 are connected and disconnected with a suitable vacuum source.
The calibration substrate 7 is provided with contact pads on its top surface (not shown) to be contacted by the tips of the needles of the probe needle arrangement 4 for calibration purposes of a measuring circuit (not shown) connected with the probe needle arrangement 4.
For overcoming the mentioned inconveniences, probe stations are equipped with load support devices 8, which allow the wafer chuck 1 to disconnect from the 4axis manipulator stage 3 or the add-on plate 2 to disconnect from the wafer chuck 1 temporarily, to bring these wafer support surfaces on the wafer chuck 1 or on the add-on plate 2 into a position which is a convenient position 9 for the operator for loading/unloading the wafer 12 and/or the calibration substrate 7 outside the chamber 6 (FIG. 1a). For that purpose, the chamber 6 is provided with a door 16 in the chamber wall.
After reconnection of the wafer-chuck 1 to the 4axis manipulator stage 3 or of the add-on plate 2 to the chuck 1 there remains a displacement 10 (FIG. 2b) of these elements to each other relative to the position before disconnection 11 (FIG. 2a) within several 10 μm, if there are used, cost effective mechanisms for carrying out these loading/unloading movements.
For several measurement purposes there are necessary both elements on the wafer chuck 1—the wafer 12 and the calibration substrate 7. The calibration substrate 7 remains normally connected with the wafer chuck 1 or with the add-on plate 2 during replacement of the wafer 12 with a new wafer 12. Loading a new wafer 12 leads always to a new position of the wafer 12 relative to the wafer stages, i.e. the wafer chuck 1 or the add-on plate 2 FIG. 2b).
It is accepted to perform an alignment of the wafer 12 relative to the probe needle arrangement 4 after loading. However the location of the calibration substrate 7 is expected to remain stable relative to the probe needle arrangement 4, so that a calibration position 15 once set can be reproduced automatically within less than a 5 μm error after loading a new wafer 12. Due to the big displacement of the wafer chuck 1 relative to the 4axis manipulator stage 3 or of the add-on plate 2 relative to the wafer chuck 1 there is required a new teaching of the calibration position of the calibration substrate 7.
This new teaching of the position of the calibration substrate 7 can be realized manually by the operator by visually controlling the reproducing of the calibration position with a suitable microscope. This is very time consuming and not precise enough since the needles of the probe needle arrangement 4 must be connected with contact pads on the calibration substrate 7 with a very high exactness. Only with such a precision the necessary calibration can be performed.